Super-capacitors

In recent several years, super-capacitors are attracting more and more attention because of their high capacitance and potential applications in electronic devices. The performance of super-capacitors with MWCNTs deposited with conducting polymers as active materials is greatly enhanced compared to electric double-layer super-capacitors with CNTs due to the Faraday effect of the conducting polymer as shown in Fig. 9.18 (Valter et al., 2002). Besides those mentioned above, polymer/ CNT nanocomposites own many potential applications (Breuer and Sundararaj, 2004) in electrochemical actuation, wave absorption, electronic packaging, selfregulating heater, and PTC resistors, etc. The conductivity results for polymer/CNT composites are summarized in Table 9.1 (Biercuk et al., 2002). 

Aprotic electrolytes of an adequate high conductivity are necessary for lithium batteries and super capacitors. Therefore, recently, much industrial research has been done in this area and highly sophisticated electrolyte systems have been developed (e.g. [64]). The supporting electrolytes for aprotic solvents generally are more or less expensive and toxic. After the reaction, their separation and recycling is inevitable and frequently needs considerable efforts. 

The resorcinol-formaldehyde polymers have been used to prepare highly porous carbon materials, by controlled pyrolysis in an inert atmosphere [144,154], The microstructure of the carbon is an exact copy of the porous polymer precursor. Poly(methacrylonitrile) (PM AN) PolyHIPE polymers have also been used for this purpose. These monolithic, highly porous carbons are potentially useful in electrochemical applications, particularly re-chargeable batteries and super-capacitors. The RF materials, with their very high surface areas, are particularly attractive for the latter systems. 

Capacitors can be polarized or non-polarized, depending on the - dielectric. Non-polarized devices have dielectrics consisting of ceramics or polymers (such as polystyrene, polyester, or polypropylene). They are normally box-shaped and their capacity is usually in the range from pF to pF, the maximum voltage up to 1000 V. Polarized capacitors are electrochemical devices the dielectric is an anodic oxide of A1 (pF to 100 mF, potentials up to 1000 V), Ta (capacities pF to 100 pF, potentials up to 20 V), or Nb (- electrolytic capacitor) or a double layer (- supercapacitor, capacities up to some 10 F and potentials up to 2.5 V or 5 V). Aluminum electrolytic capacitors are normally of cylindrical shape with radial or axial leads. Tantalum capacitors are of spherical shape and super capacitors form flat cylinders. 

Raw materials containing 30% of SWNTs have been also used for the preparation of super capacitor electrodes after mixing with poly(vinyhdene chloride) (PVDC) and pyrolysis of the mixture at 1000 C. The authors attribute the high capacitance value of 180 F/g measured in 7.5 M KOH to the SWNT material [103]. However, in reality, the carbonization of PVDC at 1000°C produces microporous carbon [107], and as a consequence the high capacitance values are certainly because of the carbon obtained from the polymeric binder but not of SWNTs. 

Jiang, Q., Qu, M.Z., Zhou, G.M., et al. (2002). A study of activated carbon nanotubes as electrochemical super capacitors electrode materials. Mater. Lett., 57, 988-91. 

Nanoporous carbon and its electrochemical application to electrode materials for super capacitors in relationship to the key role nanoporous carbons have played in the purification of liquids and the storage of energy... 

In this section, the main characteristics of alternative electrical energy storage systems, such flywheels and super capacitors, are described, as these devices might play an important role in the development of vehicles powered by electric propulsion systems. 

Super capacitors have relatively high specific power and low specific energy, like flywheels. For this reason both these devices can be used as energy storage... 

Several research studies have been carried out with super capacitors playing a significant role in the management of energy on board [41—45]. 

Super capacitors can be used in addition to batteries in electric vehicles. In Fig. 5.22, a simplified scheme for a vehicle equipped with battery and super capacitors is shown [4]. The super capacitors allow the kinetic energy of the vehicle to be recovered when the vehicle slows down, increasing the availability of power peaks during rapid accelerations. In this way, an optimal management of batteries can be accomplished. 

Super capacitors and flywheels, as high power storage systems, can be used in a wide range of hybrid vehicles, combined with one or more power energy devices. 

Fig. 5.22 Configuration of electric vehicle equipped with super capacitors and batteries...

Other configurations of fuel cell vehicles can be realized combining the advantages of different types of storage systems. As an example, the Fig. 5.24 shows the combination of rechargeable batteries with a super capacitor system. In this case, a three-way converter is required to connect the two storage systems with the fuel cell stack and interface the different voltage versus current characteristics of the devices interconnected [46]. 

Fig. 5.24 Configuration of fuel cell vehicle equipped with rechargeable batteries and super capacitors...

Conway BE (1999) Electrochemical Super Capacitors. Kluwer Academic/Plenum Publisers, New York... 

Application of Nanotextured Carbons for Super capacitors and Hydrogen Storage... 

First, a straight fuel cell power train is advocated and, secondly, an electric hybrid power train, combining a fuel cell system and an electric energy storage device such as a super capacitor or a battery is proposed (see Fig. 8.14). The advantage of the straight set-up is its simpler structure, however in this concept the fuel cell system... 

Fig. 8.15 (A) Schematic of concept car NECAR 4 representing a fuel cell vehicle with a PEM system operated with liquid H2, ([22]) (B) Passenger car with hybrid fuel cell and super capacitor power train [28],...

The most logical approach is to look for alternatives to the conventional Pb negative plates or to reduce their contribution to battery operation. Carbon super-capacitors have been chosen for this purpose. Probably, there are other methods, too. 

Axion Power International (USA) has replaced entirely the negative plates of VRLABs with carbon super-capacitor plates [80]. CSIRO and Furukawa Battery have developed and tested a battery (called the Ultra Battery) with negative plates comprising half regular lead plate and half carbon super-capacitor plate [81]. 

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